The present invention relates to a method of manufacturing aluminum conductor wires. More particularly, it relates to a method of manufacturing aluminum wires for communication cables which have excellent mechanical and electrical properties.
In the past, copper wires were exclusively used as conductors for communication cables, conductors for insulated electric wires, and the like. However, the rise and fluctuations of the price of copper throughout the world in recent years caused by the instability of the supply system of copper have prompted the transition from copper to aluminum and the movement to the use of aluminum wires and aluminum alloy wires for such conductors has rapidly come about.
Under the circumstances, the aluminum wires and aluminum alloy wires used for these purposes have come to be required to possess properties equal to those that have been possessed by copper wires used in the past.
Plastic-insulated communication cables using copper for their conductors are usually manufactured by the steps shown in FIG. 1(a).
Of these steps, the steps (A) through (E) make up a completely tandemized line, with the drawing machine, continuous annealing machine, extruder, cooling pipe and take-up machine arrayed in series. This manufacturing line is commonly referred to as the tandemized line for communication cables. Several hundreds or several thousands of wires manufactured in this way are stranded together to be completed as the so-called communication cable, so that an exceedingly high productivity is naturally required of this tandemized line. In order to realize the use of aluminum for communication cables, therefore, it is indispensable to develop an aluminum material and a manufacturing method which are suitable for this modernized continuous manufacturing line.
Compared to copper, aluminum generally has a strength of about one-third and an electrical conductivity of about 60%. This lower electrical conductivity can be compensated for by increasing the diameter of the conductor, but this will not be entirely satisfactory from the view-point of strength.
If the object is merely to improve strength, then it is conceivable to omit the continuous annealing step, i.e., step (B) in the manufacturing line of FIG. 1(a) and use it as a completely hard material, or to add a complete annealing step at 300.degree. to 400.degree.C before the step (A) of the manufacturing line of FIG. 1(a) and instead omit the continuous annealing of step (B), thus using it as a 3/4 H material.
However, the properties required of communication cables are not only strength. At the time of manufacture, burying, jointing, and installation, they are required to have sufficiently good properties of elongation, resistance to bending, flexibility, etc. Furthermore, as inherent requirements of communication cables, satisfactory properties with respect to mutual capacitance, capacitance unbalance, cross-talk, attenuation, etc., are also required.
The conventional manufacturing method of FIG. 1(b) will now be described in further detail. In manufacturing the wire, usually a supply wire of a diameter of approximately 2 mm drawn by a breakdown machine is drawn by the drawing machine of the tandemized line (Step (A) in FIG. 1(b)) to a final size or a size about 10 to 30% larger in cross-sectional area than the final size. This wire is subjected to the continuous annealing machine (Step (B) in FIG. 1(b)) and made a 1/4 H material, after being further drawn by a following 1-die drawing machine when necessary. Before and after this continuous annealing machine, it is a common practice to provide a trough for washing off the drawing lubricant and to provide a cooling trough. In the meantime, the conductor is subjected to bending of at least 10-odd turns by going through guide rollers or the like. Then the conductor is further subjected to the plastics extruder and goes through the cooling pipe which is as long as 10 to 20 m. It is taken up by a take-up machine after going through dancer rollers, tension helpers (self-driving rollers), etc. During these steps the conductor usually comes to have a total extension of 20 to 100 m (depending on the design and arrangement of the apparatus, number of turns wound on dancer rollers, etc.), and it is subjected to a bending of several tens of turns. Communication cables having copper conductors are also manufactured generally by similar manufacturing steps. If aluminum conductors are used, however, still greater consideration has to be paid to the trough for washing off the lubricant, continuous annealing machine, etc., so that the manufacturing line inevitably becomes longer and more complicated. Perhaps this will easily be understood from the following well-known facts.
Compared to copper, the phenomenon of sticking at the time of drawing is more liable to take place with aluminum, so that a lubricant of a high viscosity which is different from that used on copper is required. In addition, fine dust is exceedingly liable to be produced when drawing aluminum. Also, the oxide film formed on the surface of aluminum makes electric continuous annealing extremely difficult.
Furthermore, the substantial differences between copper and aluminum used as conductors is evidently noticeable even in the wire-setting work conducted at the start of manufacture. Ordinarily, the supply wire of a diameter of about 2 mm has a tensile strength of approximately 45 Kg/mm.sup.2 in the case of copper, and approximately 18 Kg/mm.sup.2 in the case of aluminum for electrical purposes. However, if this conductor is drawn by the drawing machine of a tandemized line to a desired size (about 0.3 - 0.8 mm in most cases for communication cables), copper is hardened only about 2 - 3 Kg/mm.sup.2, as seen from the property of work-hardening by drawing shown in FIG. 2, while aluminum and aluminum alloys become hardened to as much as about 10 Kg/cm.sup.2 and show a remarkable degradation in elongation, becoming exceedingly brittle. Usually the continuous annealing machine is not put into operation at the time the wire-setting work is done. As much as several tens of meters of this brittle conductor has to be conveyed to the take-up machine after receiving repeated bending work for several tens of times. This requires very careful attention and skill in comparison to that required in the case of a copper conductor. That is to say, a flexibility which can withstand the repetition of bending work is strongly demanded for the wire-setting operation at the commencement of manufacture.
Next, an important property required at the time of manufacture is strength against line tension. This is a requirement that must be said to be quite natural if it is considered that the conductor has to travel through the tandemized line having a total length of several tens of meters as already described at a speed of several hundreds of meters per minute, passing through the cooling trough, washing trough, plastics extruder, etc. More particularly, the smaller the conductor size is, the more important this property becomes.
In the course of making a cable of strands manufactured in the above-described way, properties for easy handling, such as flexibility and resistance to bending, becomes necessary again. A hardness of a suitable degree will also become necessary at the time of conductor pointing and splicing.
On the other hand, the properties of conductors are of extreme importance also from the viewpoint of the requisite properites of communication cables. In the case of communication cables, several hundreds or several thousands of individual strands are stranded together to complete one cable. It is well known that the condition of this stranding of strands has a great influence on the quality of the communication cable, especially capacitance unbalance inside quads and cross-talk inside quads. That is to say, if the conductors have a high rigidity and the adaptability among individual strands is poor, the capacitance unbalance inside quads becomes remarkably degraded and the use of aluminum in place of copper will become meaningless.
For the reasons mentioned above, the properties which aluminum is required to possess at the present time to meet the manufacturing conditions and the requirements of communication cables are said to be as follows: If the diameter of the completed conductor is 0.8 mm. then tensile strength should be 9.5 Kg/mm.sup.2 or more, elongation 3% or more, electrical conductivity 61.0% or more. If the conductor is of a comparatively small size such as 0.65 mm, 0.5 mm, etc., tensile strength should be 12.0 Kg/mm.sup.2 or more, elongation 3% or more, and electrical conductivity 60.0% or more.
These requisite properties are the properties equivalent to those of the so-called 1/4 H material, and it is possible to satisfy these properties by giving a cold working of about 20% after continuous annealing or by using some suitable aluminum alloy in place of aluminum for electrical purposes. As has already been mentioned, however, the wire setting operation at the commencement of manufacture or at the time of wire breaking is extremely difficult. This has been a big obstacle in blocking the way to mass production of communication cables of aluminum conductors.
The measure that has been generally taken in the case where the material is subjected to intense working and becomes brittle as mentioned above is to give the material an intermediate annealing treatment at about 300.degree. to 500.degree.C after a suitable working, thereby making it a soft material and thereby preventing it from becoming brittle. The present inventors also carried out an intermediate annealing in a similar way after a cold working of about 90% to make the material a soft material and then manufactured communication cables of aluminum conductors by the steps shown in FIG. 1(b). It was then found that although an improvement was observed in workability, the overall properties of strength, elongation and electrical conductivity were remarkably lower than those of the conductors manufactured without the intermediate annealing and were not in conformity with the aforementioned property requirements.